Reference is hereby made to our copending application Ser. No. 422,050, filed Dec. 5, 1973, now U.S. Pat. No. 3,873,895.
This invention relates generally to the field of electrostatics and particularly to a technique for charging dielectric surfaces to high voltage levels.
One application for charging devices is in xerography in which a photoconductive dielectric surface is charged with positive or negative charges, then exposed to an object whereby the imagewise discharging of the photoconductor leaves a latent electrostatic image of the original object on the surface.
One present technique for applying electrostatic charge to a dielectric surface is by means of a corona discharge. In such an arrangement, a corona generating device is placed near the dielectric surface to be charged and ions generated around the corona wire flow to the dielectric surface because of potential difference between the wire and the surface. Further details regarding present corona charging devices and their relation to xerography are available in "Xerography and Related Processes" by Dessauer and Clark, published in 1965 by The Focal Press. The relevant disclosures in this book are incorporated in this specification by reference.
In corona charging devices typical of the prior art (see FIG. 1), a fixed voltage level is applied to a corona wire and charge is deposited and accumulated on the nearby dielectric surface or plate. The wire voltage must be high enough that the electric field surrounding the wire ionizes the air in the immediate vicinity. This minimum voltage, which is required to initiate a corona current, is called the threshold voltage (V.sub.O). Another factor is that the corona current is dependent upon the voltage difference or potential difference between wire and plate. Thus, if a corona wire is used to charge a dielectric plate, the current is determined by the voltage difference between the wire and the plate. If a voltage V.sub.w is applied to the wire, an initial current I.sub.w is drawn. This current will charge the dielectric plate and thereby reduce the potential difference between the wire and the plate. The corona current then decreases along a current-voltage curve until the voltage difference between the wire and the plate surface is reduced to the threshold voltage V.sub.O as a limit. In other words, the corona current asymptotically approaches zero.
The voltage on the plate, V.sub.p, increases with time during the charging and asymptotically approaches the value of the wire voltage less the threshold voltage. That is, the limit V.sub.p = V.sub.w - V.sub.O.
For wire-to-plane coronas, the maximum voltage which corona is finally capable of applying to dielectric plates (V.sub.p max) is determined by the maximum voltage which can be applied to the wire, which is essentially the sparking or arcing voltage V.sub.s. Thus, by extending the foregoing equation to its limit, V.sub.p max = V.sub.s - V.sub.O.
By means of the present invention, the voltage limits inherent in such prior art charging methods are removed.